The language chooser in the OPAC at the bottom produces wrong HTML code: <a class="dropdown-item"<a href="#" tabindex="-1" class="menu-inactive" role="menuitem"> To see this, you need to at least one additional language installed and also have it activated in the opaclanguage* parameters.
https://git.koha-community.org/Koha-community/Koha/src/commit/a57278f39b8c5b7a82b21671f266dbf32e2d41a0/koha-tmpl/opac-tmpl/bootstrap/en/includes/masthead-langmenu.inc#L31
*** This bug has been marked as a duplicate of bug 27881 ***